The present invention generally relates to a continuously variable transmission, and more specifically to an apparatus and method for preventing hunting between ranges in a continuously variable transmission.
Many work machines, particularly earth working machines, use a continuously variable transmission to drive traction wheels or tracks which propel the work machine. Typically, a hydrostatic transmission, consisting of a variable speed hydraulic pump and a hydraulic motor, provides a continuously variable speed output to the wheels or tracks of the work machine. In particular, the speed output can be continuously varied by controlling the displacement of the hydraulic pump.
In order to operate over a wide range of operating conditions, the engine torque is split between the hydrostatic transmission and a mechanical transmission. The mechanical transmission has a number of transmission ranges corresponding to different operating speeds of the work machine. The combination of the hydrostatic transmission and the mechanical transmission allows a continuously variable transmission to operate over a wider range of speeds than is possible using the hydrostatic transmission alone.
A drawback to using a number of transmission ranges is that the transmission can xe2x80x9chuntxe2x80x9d or continuously shift between ranges. As the transmission hunts, performance of the transmission can be degraded by the numerous toque interruptions which accompany each shift. Conventional transmissions employ timer based anti-hunt routines which inhibit a downshift subsequent to an upshift during a timer period after the upshift. Similarly, anti-hunt routines inhibit an upshift subsequent to a downshift during the timer period after the downshift. In addition to timers, the upshift and downshift point are selected to build in a natural hysteresis. A second downshift point is selected which allows a downshift ignoring the timer, but this shift point is associated with a deep engine lug (which is undesirable in continuously variable transmission applications).
In addition, most conventional transmissions also employ a torque converter between the engine and the transmission. The torque converter provides a fluid coupling which absorbs shocks during shifting. Moreover, the torque converter allows the engine to remain within its operating range as transmission loads vary considerably. Thus, the torque converter allows the engine to continue operating effectively if the output load increases rapidly during the timer period.
For efficiency reasons, it is unnecessary and undesirable to use a torque converter in conjunction with a continuously variable transmission. A drawback to eliminating the torque converter is that when using a timer based anti-hunt routine, a rapid increase in transmission load during the timer period may stall the engine. Thus, an anti-hunt routine for a continuously variable transmission must be able to rapidly respond to changes in transmission load and cannot use timer based anti-hunt routines, while still shifting near the shift point (near a synchronous speed) in order to allow the variable displacement hydraulic pump to be the sole device for changing transmission ratio.
What is needed therefore is a method and apparatus for preventing hunting between ranges in a continuously variable transmission which overcomes the above-mentioned drawbacks.
In accordance with a first aspect of the present invention, there is provided a transmission assembly having a continuously variable transmission driven by the engine, a mechanical transmission coupled to the continuously variable transmission for selecting a first gear range and a second gear range and a controller for receiving operator inputs and generating commands operable to control an output of the continuously variable transmission. The controller executes a first shift between the first gear range and the second gear range at a shift point in response to the commands. The controller places the transmission in a first mode of operation where a second shift from the second gear range to the first gear range is inhibited. The controller places the transmission in a second mode of operation where the second shift from the second gear range to the first gear range is allowed when the output progresses beyond the shift point by a first value or the command falls past the shift point by a second value. The controller executes the second shift from the second gear range to the first gear range when the transmission is in the second mode of operation and the output reaches the output at the shift point.
In accordance with a second aspect of the present invention, there is provided a method of controlling a transmission assembly having a continuously variable transmission driven by the engine, a mechanical transmission coupled to the continuously variable transmission for selecting a first gear range and a second gear range, and a controller for receiving operator inputs and generating commands operable to control an output of the continuously variable transmission. The method includes the steps of executing a first shift between the first gear range and the second gear range at a shift point in response to the commands and placing the mechanical transmission in a first mode of operation where a second shift from the second gear range to the first gear range is inhibited in response to the executing step. The method further includes the step of placing the mechanical transmission in a second mode of operation where the second shift from the second gear range to the first gear range is allowed when the output progresses beyond the shift point by a first value or the command falls past the shift point by a second value. The method yet further includes the step of executing the second shift from the second gear range to the first gear range when the mechanical transmission is in the second mode of operation and the output reaches the output at the shift point.